Diff comments do not show up on changes tab with parallel view, but are visible on discussions tab

In this MR, I added a few comments while using parallel view + hiding whitespace changes, and the comments are never visible on the diffs view: Screen_Shot_2018-08-26_at_09.03.38

However, they are visible in the discussions tab: Screen_Shot_2018-08-26_at_09.05.05

/cc @timzallmann @andr3 this seems like a P1 to me.

Use this URL to test: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21288?w=1&view=parallel